Field
The present disclosure relates to an organic light emitting diode display driven through a digital driving method and a method for driving the same.
Discussion of Related Art
Because an organic light emitting diode display (hereinafter, referred to as “OLED display”) is a self-emission display device, the OLED display may be manufactured to have lower power consumption and thinner profile than a liquid crystal display which requires a backlight unit. Further, the OLED display has advantages of a wide viewing angle and a fast response time and thus has expanded its market while competing with the liquid crystal display.
The OLED display is driven through an analog voltage driving method or a digital driving method and using grayscale values of an input image. The analog voltage driving method adjusts a data voltage applied to pixels based on data gray values of the input image and adjusts a luminance of the pixels based on a magnitude of the data voltage, thereby representing grayscale of the input image. The digital driving method adjusts an emission time of the pixels based on the data gray values of the input image, thereby representing grayscale of the input image.
As shown in FIGS. 1 and 2, the digital driving method time-divides one frame into a plurality of subframes SF1 to SF6. Each subframe represents one bit of input image data. As shown in FIG. 1, each subframe may include a writing time ADT, during which data is written on pixels, and an emission time EMT, during which the pixels emit light. As shown in FIG. 2, each subframe may further include an erase time ERT, during which the pixels are turned off, in addition to the writing time ADT and the emission time EMT. The emission times of the subframes may have different lengths. However, all of the subframes are the same as one another in a scan direction for writing the data and an erase direction for turning off the pixels. In addition, a writing speed for writing the data and an erase speed for turning off the pixels in each subframe are the same as each other. Therefore, the emission time of the same subframe is uniform irrespective of a position of the display panel.
Because the emission time of the same subframe is uniform, there should be negligible luminance deviation across pixels irrespective of the position of the pixel within the display panel. However, as shown in FIG. 3, because variations IR drop resulting from varying line resistances occur in the display panel, a high potential power voltage EVDD varies depending on a spatial position of the display panel to thereby generate the luminance deviation. The luminance implemented in the display panel decreases as the display panel is far from an input terminal of the high potential power voltage EVDD.
In the analog voltage driving method, a driving thin film transistor (TFT) is driven in a saturation region. As shown in FIG. 4, the saturation region is a voltage region, in which a drain-source current Ids does not substantially change as a function of a drain-source voltage Vds of the driving TFT, and is positioned on the right side of the Vds-Ids plane (for values of Vds above a threshold). In other words, in the saturation region, the drain-source current Ids does not change although the high potential power voltage EVDD (i.e., the drain-source voltage Vds of the driving TFT) changes.
On the other hand, in the digital driving method, the driving TFT is driven in an active region, so as to reduce power consumption. As shown in FIG. 4, the active region indicates a voltage region, in which the drain-source current Ids changes depending on the drain-source voltage Vds of the driving TFT, and is positioned on the left side of the Vds-Ids plane (for values of Vds below a threshold). In other words, in the active region, the drain-source current Ids changes depending on changes in the high potential power voltage EVDD (i.e., the drain-source voltage Vds of the driving TFT).
For this reason, the luminance deviation resulting from the IR drop is more of a problem in the digital driving method than the analog voltage driving method.